Mycorrhiza—Saprotroph Interactions and Carbon Cycling in the Rhizosphere

Labile carbon (C) inputs in soils are expected to increase in the future due to global change drivers such as elevated atmospheric CO₂ concentrations or warming and potential increases in plant primary productivity. However, the role of mycorrhizal association in modulating microbial activity and soil organic matter (SOM) biogeochemistry responses to increasing below-ground C inputs remains unclear. We employed ¹⁸O–H₂O quantitative stable isotope probing to investigate the effects of synthetic root exudate addition (0, 250, 500, and 1000 μg C g soil⁻¹) on bacterial growth traits and SOM biogeochemistry in rhizosphere soils of trees associated with arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) fungi. Soil respiration increased proportionally to the amount of exudate addition in both AM and ECM soils. However, microbial biomass C (MBC) responses differed, increasing in AM and decreasing in ECM soils. In AM soils, exudate addition increased taxon-specific and community-wide relative growth rates of bacteria, leading to enhanced biomass production. Conversely, in ECM soils, relative growth rates were less responsive to exudate addition, and estimates of MBC mortality increased with increasing exudate addition. In the AM soils, aggregated bacterial growth traits were predictive of soil respiration, but this relationship was not observed in ECM soils, perhaps due to substantial MBC mortality. These findings highlight the distinct responses of bacterial communities in AM and ECM rhizosphere soils to exudate addition. Considering that microbial products contribute to the formation of stable soil organic carbon (SOC) pools, future increases in labile exudate release in response to global change may consequently lead to greater SOC gains in AM soils compared to ECM soils.